The method you are using will not work. You are referencing an undeclared variable sold in this snip: {sold: !sold}. If anything, you'd want to do {sold: !this.sold}, but this within a findById is a query and not the model. You would have to use findById for the book you want, update it manually (book.sold = !book.sold) then save it.

You could create a static method on your model to accomplish all this.